1. Field of the Invention
The present invention relates to vehicle brake systems of the type employing both spring brake actuators and service brake actuators, and more particularly, to a brake system where fluid which has been discharged from the spring brake actuators to effect spring brake application is exhausted through the service brake actuators to momentarily effect brake application through the service brake actuators.
2. Prior Art
Many heavy-duty, over-the-road vehicles are provided with brake systems including both service and spring brake actuators which selectively effect brake application. The service and spring brake actuators may be arranged to brake separate vehicle wheels, or may be arranged in tandem to brake common wheels.
The service brake actuators are normally used to arrest vehicle movement. Each of the service brake actuators has a service brake chamber which, when supplied with pressurized fluid, effects a vehicle braking action.
The spring brake actuators are normally used when the system has failed. Each of the spring brake actuators has a spring brake chamber and one or more springs for effecting brake application if the fluid pressure in the spring brake chamber is below a predetermined level. When the spring brake chambers are pressurized above this level, the biasing action of the springs is overcome, and brake application is released.
It is common to mount a pair of spring and service brake actuators in tandem for operating a common brake control member. The control member extends to apply a braking force if either of two conditions obtain, namely if fluid pressure in the service brake chamber is above a first level, (typically about 4 or 5 psi), or if fluid pressure in the spring brake chamber is below a second level (typically about 50-60 psi).
Service and spring brake conduit networks respectively communicate the service and spring brake chambers with a source of pressurized fluid. Each of the conduit networks includes valving for selectively pressurizing and exhausting its associated brake chambers. When the service brake supply network is not being pressurized to effect brake application through the service brake actuators, this network is normally vented to atmosphere.
One or more quick-release valves are normally provided in the spring brake conduit network to facilitate rapid exhaustion of the spring brake chambers to atmosphere. If an emergency stop must be made at a time when the service brake supply network has failed, the quick-release valves enable the spring brake actuators to be rapidly brought into play to stop the vehicle. Since the spring brake chambers are normally pressurized at about 100-200 psi, a brief period of time (typically about 750 milliseconds) is required to exhaust the spring brake chambers to a pressure (of about 50-60 psi) where spring brake application can begin. Full spring brake application does not take place until the spring brake chambers are exhausted to ambient pressure.
Many over-the-highway vehicles have selected wheels which are provided solely with service brake actuators, while other wheels are provided with tandem spring and service brake actuators. In an emergency situation where the service brake supply network has failed and vehicle movement must be arrested, only the vehicle wheels provided with spring brake actuators can be braked. While it would be desirable to provide the nonspring-braked wheels with an emergency braking capability, no way has previously been found to achieve this result.